Your SQL bigint takes the last 8 bytes while the c# implementation takes the first 8 bytes (and reverses them because its running on little endian).
Take the proper Range of the array in C# and reverse it. Then you should be fine.
Did some coding:
System.Security.Cryptography.SHA1 c = System.Security.Cryptography.SHA1.Create();
byte[] b = c.ComputeHash(Encoding.UTF8.GetBytes("google.com"));
long value = BitConverter.ToInt64(b, 12);
value = IPAddress.HostToNetworkOrder(value);
Debug.WriteLine(value);
// writes 2172193747348806725
